The field of this invention is in connection with the end threaded and intermediate annularly insulated tubular conduits utilized in the steam injection of oil wells. In the recovery of high viscosity hydrocarbons it is common practice to supply high temperature steam to the producing formation. The steam is directed to the formation through a tubing string which is hung from the surface. The tubing string is made up of multiple lengths with threaded end connections to facilitate installation into and removal from the well. It is common practice to insulate the outer surface of the tubing to prevent heat loss and maintain the enthalpy of the steam. The well environment usually requires that this insulation be protected from abrasion and contact with liquids by an outer jacket, or casing. The tubing, insulation, and outer casing are preassembled in individual lengths to facilitate installation into and removal from the well.
Since the tubing and casing are therefore exposed to different temperatures, the linear expansion rate of the tubing and of the outer casing are different. Due to this fact, there are typically three (3) types of joints presently used which so connect the outer casing to the tubing. The joints are either hard, flexible, or slip-type. In a hard joint the tubing and casing are welded together on the body of the tubing a short distance from each end. The exposed tubing end connections are used to join adjacent lengths, and therefore the tubing becomes the principal tensile load carrying member. The high strength steel tubing, normally used in this application, has poor weldability. Leakage or tensile failure often occurs in this type of joint due to stress cracks that develop in the weld or in the heat affected zones of the tubing.
Flexible joints are often used which utilize bellows type devices to seal the annular insulation space between the tubing and outer jacket while allowing relatively free expansion of the tubing with respect to the outer casing. A potential weakness of this design is the susceptability of the relatively thin metal bellows to corrosion failure.
Slip joints have been utilized which usually consist of a piston ring or packing material arrangement. The disadvantage of this joint is the leakage potential inherent in slip joints.
An object of this invention is to provide a prestressed insulated concentric tubing and outer casing unit, and a mechanical and self sustaining combination coupling connecting the inner tubing and outer casing.
A further object of the invention is to provide a joint which is leak resistant and positively sealed.
A further object of the invention is to provide a joint having no welds which are critical to its structural integrity.
A further object of the invention is to provide a pipe length assembly which can be connected to other such utilizing standard threaded couplings.
The foregoing and similar objects, advantages and features may be achieved by the INSULATED CONCENTRIC TUBULAR JOINT ASSEMBLY for conveying heated fluids such as steam and the like from the ground source to the down hole formation as described in the following description read in conjunction with the drawings and claims.